Paper mills have for many years made extensive use, for the screening of paper making stock, of screen apparatus embodying a cylindrical perforate screen member defining supply and accepts chambers on the opposite sides thereof in a closed housing and provided with a rotor member which operates in one of the chambers to keep the screen perforations open and free from solid material tending to cling to the screen surface. Commonly, the stock or furnish is delivered to the supply chamber adjacent the end of the screen member, and the material rejected by the screen member is collected and discharged from the opposite end of the screen member.
The assignee of this invention has manufactured and sold many such screens, originally in accordance with Staege U.S. Pat. No. 2,347,716, and more recently in accordance with Martindale U.S. Pat. No. 2,835,173, the latter construction being characterized by a rotor comprising bars or vanes of air-foil section in closely spaced but non-contacting relation with the surface of the screen member. More specifically, these vanes have been moved along the screening surface at relatively low speeds, e.g. in the range of 1,250-2,500 feet per minute, with the clearance between the supply side of the screen member and the nearest portion of the vanes being in the range of 0.030-0.060 inch.
The art has experimented widely with detailed variations in screens of the above type, including variations in the vane shape and other forms of rotor, and also in the size, spacing and configuration of the perforations in the screen member. In recent years, such screens have been offered to the trade wherein the rotor is a wall member provided with multiple bumps or other offset portions over its surface for the purpose of creating localized changes in volume, and resulting agitation effects, in the annular space between the rotor and the screen member, a typical such construction being shown in Clarke-Pounder U.S. Pat. No. 3,363,759.